This invention relates generally to preamplifiers of signals from sensors having a resistance that varies in a manner related to a condition or parameter being measured, and, separately, to bias current sources.
One such variable resistance sensor is a magneto-resistive element that is positioned as a read element in a read/write head of magnetic tape and magnetic disk mass storage systems. The small resistance of such an element varies a small percentage in response to variations in a magnetic field it is detecting. In order to obtain a signal output, such an element is biased with a constant current through it or voltage across it, and then variations in the other of the current or voltage are amplified. Since this signal variation is quite small, great care must be taken in its initial amplification in order to maintain a high signal-to-noise ratio.
One such preamplifier is disclosed in U.S. Pat. No. 5,122,915 of Klein et al. (1992), the disclosure which is incorporated herein by this reference. Briefly, that preamplifier maintains a constant voltage across the magneto-resistive head element. A varying current that follows the varying resistance of the sensing element is converted into a varying voltage signal output. This voltage signal output is larger than a voltage variation that would exist across the magneto-resistive head element with a constant biased current applied.
However, the gain-bandwidth product of preamplifiers following the design in U.S. Pat. No. 5,122,915 is somewhat limited, unless the overall gain is made less than desired or other sacrifices are made. This is believed due to the existence of parasitic capacitance in combination with an output resistor that converts the varying head current into a varying voltage output. This bandwidth limitation is of no concern in many applications but, for other applications, it is desired to remove such a limitation. Therefore, it is the primary object of the present invention to improve the preamplifier circuit described in U.S. Pat. No. 5,122,915, particularly to provide an increased gain-bandwidth product, and also to further decrease any distortion present in the output signal.
It is another object of the present invention to provide an improved current source, for use in such a preamplifier or in other more general applications, that exhibits a reduced level of noise.